Urethane polymers have in the past been modified to modulate their functionality by endcapping some or all of the isocyanate groups with a variety of organosilanes to yield silane end-capped urethane polymers containing minimal or no isocyanate groups. Silane-endcapped urethane sealants frequently exhibit insufficient flexibility to be useful in applications requiring considerable extension and compression. To overcome these problems, U.S. Pat. No. 4,645,816 to Pohl and Osterholtz described a novel class of room-temperature, moisture-curable, silane-terminated polyurethanes bearing terminal isocyanate groups reacted with a silane monomer having one dialkoxysilyl group and an organo-functional group with at least one active hydrogen. The polymers were crosslinked to produce elastomeric networks with improved flexibility. Another approach to reducing the crosslinking density of the cured elastomers, is to use secondary aminosilanes with bulky substituents on the nitrogen as silane endcappers, preferably reacting all free isocyanate endgroups with these secondary amino silanes.
The use of difunctional silanes and/or sterically hindered silanes, typically employing amine reactivity for the endcapping of the urethane prepolymers, suffers from several drawbacks. The secondary amine containing silanes are slow to react with the urethane prepolymer while polymer endcapped with dialkoxysilyl functional silanes are typically very slow to cure. Particularly the formation of urea, which is experienced when using amino silanes, leads to a meaningful increase in viscosity of the prepolymer, potentially resulting in processing problems and application restrictions. A high viscosity silane terminated prepolymer would in particular hinder the formulation of a low viscosity coating composition. Organic solvents could be used to thin a coating formulation based on the aminosilane terminated prepolymer and would allow application of the material by spraying, dipping or brushing. However, polymers requiring greater amounts of solvents are less desirable because of volatile organic compound emissions, flammability or health perspectives as examples.
Employing trialkoxysilyl groups as endcappers for silylated precursors intended to have considerable flexibility requires forming extended polymer chains with a significantly high average molecular weight to balance out the crosslinking density inherent in the use of the trifunctional endcapper. This polymer structure has led to increased viscosity of the eventual product to unacceptably high levels particularly when amino silane endcappers are used to build urea bonds. The synthesis routes to build high molecular weight isocyanate-terminated polyurethanes using di- or polyisocyanates and conventional polyether polyols have exhibited the problem of low to negligible isocyanate residual functionality of the urethane prepolymer before silane endcapping. Hence, synthesis of these types of systems may be not feasible, and/or these systems may offer unacceptable cure profiles and mechanical properties.
Silyl-terminated polyurethanes (STPU) prepared by directly capping polyether polyols is well known in the art. However, sealants made from the direct capping of polyether polyols have poor adhesion to many organic and inorganic substrates. Thus, there remains a need for silylated polyurethane polymers that offer improved mechanical and adhesive properties.